Four wheel steering system

ABSTRACT

A closed hydraulic system (22 or 200) is provided for transmitting steering movement to a rear wheel steering assembly (16) in response to steering movement of a front wheel steering assembly (14). The front wheel steering assembly is controlled by a steering wheel (18) and a power steering assembly (20). The closed hydraulic system (22) includes a pump assembly (50) and an actuator assembly (52) connected together by conduits to define closed fluid flow branches (104,106) for moving a piston (90) in the actuator assembly in response to movement of a piston (72) in the pump assembly. In one embodiment (22) movement of the actuator piston is transmitted to the rear wheel steering assembly via a cam assembly which effects same direction rear wheel steering for rather small front wheel steering angles and which effects opposite direction rear wheel steering for greater front wheel steering angles. In another embodiment (200), the actuator piston (214) is connected directly to the rear wheel steering assembly. Both embodiments include a make-up valve (66) connected to an accumulator functioning as a reservoir to replace system fluid lost by leakage. The system embodiment (200) is provided with a valve for reversing fluid flow from the pump to the actuator to enable the actuator to provide same and opposite direction rear wheel steering relative to the front wheels.

FIELD OF INVENTION

The present invention relates to four wheel steering. More specifically,the invention relates to apparatus for transmitting steering movement toa rear wheel steering assembly of a vehicle in response to steeringmovement of a front wheel steering assembly.

DESCRIPTION OF THE PRIOR ART

The purpose of four wheel steering is of course to reduce vehicleturning radius and/or to improve high speed performance of the vehicle.It is known to employ electrical, electrohydraulic, hydromechanical, ormechanical apparatus to transmit steering movement to rear wheelsteering assemblies of vehicles in response to steering movement offront wheel steering assemblies. In practice, implementation of suchapparatus has been expensive, has been somewhat unreliable, and/or hasprovided less than optimum performance. Further, hydromechanical andmechanical apparatus have been difficult to package into availablevehicle space and have been difficult to protect against the harshenvironment in which vehicles operate.

In a known four wheel steering system, steering movement of a frontwheel steering assembly is transmitted to a rear wheel steering assemblyvia a mechanical apparatus in the form of a rotatably mounted shaftmeans extending longitudinally of the vehicle between the front and rearwheel steering assemblies. The rotatably mounted shaft means sensessteering movement of the front wheel steering assembly and alsotransmits powers for steering the rear wheel steering assembly. It isalso known to convert this mechanical apparatus to a hydromechanicalapparatus by connecting the output of the rotatably mounted shaft to avalve which ports pressurized hydraulic fluid from a motor driven pumpto a power cylinder operative to transmit steering movement to the rearwheel steering assembly. It is also known to provide such mechanical andhydromechanical apparatus with a geared transmission operative toreverse the output direction of the rotatably mounted shaft means whenthe steering angle of the front wheel steering assembly exceeds apredetermined amount, thereby changing the steering direction of therear wheels relative to the front wheels from the same direction toopposite direction.

SUMMARY OF THE INVENTION

An object of this invention is to provide inexpensive and reliableapparatus for transmitting steering movement to a rear wheel steeringassembly.

Another object of this invention is to provide such an apparatus whichis also readily packaged into available space of vehicles.

Another object of this invention is to provide simple, inexpensive, andreliable means for reversing the steering angle of the rear wheels whenthe steering angle of the front wheels exceeds a predetermined angle.

According to a feature of the invention, a steering apparatus for awheeled vehicle having front and rear dirigible wheel assembliescomprises first means for transmitting left and right steering movementto a front wheel steering assembly in response to a steering wheel beingturned left and right from a neutral position to effect left and rightsteering angles of the front wheels; and second means for transmittingsteering movement to a rear wheel steering assembly in response tosteering movement of the front wheel steering assembly. The second meansis characterized by a closed hydraulic system having first and secondfluid flow branches comprising: a fluid displacement assembly operativeto effect fluid flow in both branches in response to steering movementof the front steering assembly; a hydraulic actuator assembly operativeto effect steering movement of the rear wheel steering assembly inresponse to the fluid flow, the fluid in the branches alternatelyincreases from a low pressure to an operational pressure in response toalternate steering movement of the front wheel steering assembly; asource of fluid under pressure less than the operational pressure in theclosed branches; and a control assembly including a valve assemblyhaving a first port connected to the fluid source, second and thirdports respectively connected to the first and second branches, a valvingmember movable to a first portion in response to the neutral position ofthe steering wheel and movable to at least a second position in responseto a non-neutral position of the steering wheel, and the valving memberfirst position communicating the first port with the second and thirdports and the second position blocking at least the communication to theport connected to the branch increasing to operational pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

A steering system according to the present invention is shown in theaccompanying drawings in which:

FIG. 1 is a schematic plan view of a steering system of the inventiondisposed between front and rear wheel steering assemblies of a vehicle;

FIG. 2 is detailed schematic view of components in the system of FIG. 1;

FIGS. 3A, 3B are relief views of a component in FIG. 2;

FIG. 4 is detailed schematic of an alternative embodiment of the systemof FIGS. 1 and 2;

FIG. 5 illustrates an alternative control logic from the system of FIG.4.

DETAILED DESCRIPTION OF THE DRAWINGS

The schematic plan view of FIG. 1 illustrates a steering system 10 for awheeled vehicle having a body and chassis represented by phantom line12, and front and rear steerable wheel assemblies 14, 16. The systemincludes a steering wheel 18, a power steering assembly or first means20 for transmitting steering movement of the steering wheel to the frontwheel steering assembly, and a closed hydraulic system or second means22 for transmitting steering movement of the front wheel steeringassembly to the rear wheel steering assembly.

The front wheel steering assembly 14 includes front wheels 28 eachhaving a steering knuckle arm 30 pivotally connected together by tie rodends 32a, 32b of a tie rod or ram 32 disposed for to-and-fro movementtransverse to the longitudinal direction of the vehicle in known manner.The tie rod or ram 32 forms part of apparatus 20 and is discussedfurther hereinafter. The rear wheel steering assembly 16 includes rearwheels 34 each having a steering knuckle arm 36 pivotally connectedtogether by tie rod ends 38a, 38b of a tie rod or ram 38 disposed forto-and-fro movement transverse to the longitudinal direction of thevehicle.

Assembly 20 may be any of several well known mechanical non-power orpower steering assemblies which transmit steering movement to the frontwheel steering assembly in response to steering movement of the steeringwheel. Apparatus 20 is preferably of the power steering type and, asillustrated therein, is of the well known hydraulic type including asource of pressurized hydraulic oil or fluid provided by a pump 42 inknown manner, a sump 44, and a servo valve 46 for connecting orcommunicating the source and sump with opposite sides of an unshownpiston disposed in a power cylinder 48. Tie rod ends 32a, 32b extendfrom opposite sides of the piston and move together in to-and-fro motionin response to alternate porting or communication of opposite sides ofthe unshown power cylinder piston with the source and sump by valve 46.

Closed hydraulic system 22 includes a pump assembly 50, an actuatorassembly 52, conduits or passages 54,56 respectively connected at oneend to ports 58a,60a defined by end caps 58,60 of the pump assembly andrespectively connected at the other end to ports 62a,64a defined by endcaps 62,64 of the actuator assembly. System 22 also includes a make-upvalve assembly 66 and an accumulator assembly 68 providing a source oflow pressure fluid.

Pump assembly 50 includes a cylinder portion or center housing member 70secured to the vehicle chassis and defining a cylindrical bore 70ahaving an opening 70b, a reciprocal piston member 72 slidably disposedin the bore and having ends 72a,72b, the end caps 58,60, and a lever arm74 mounted for pivotal movement intermediate its ends 74a,74b on a pin76 fixed relative to the pump assembly housing. Lever end 74a includes aroller 78 received in a piston recess 72c and end 74b is pivotallysecured to tie rod end 32b by a pin 80. Piston ends 72a,72b and end caps58,60 respectively define variable fluid volumes 82,84 sealed from eachother and cylinder opening 70b by dynamic seals 86 which may be of theO-ring type. Alternatively, the piston ends may be sealed by roll typediaphragms such as disclosed in co-pending U.S. application Ser. No.138,762, filed 12-28-87 and incorporated herein by reference. The endcaps are secured and sealed to the housing in known manner, e.g., byunshown screws and gaskets.

Actuator assembly 52 includes a cylindrical portion or center housingmember 88 secured to the vehicle chassis and defining a cylindrical bore88a, a reciprocal piston member 90 slidably disposed in the bore, andhaving ends 90a,90b, the end caps 62,64: springs 92,94 for centering thepiston axially in the bore 88a, and a portion 38c of tie rod 38intermediate tie rod ends 38a,38b. Piston ends 90a, 90b and end caps62,64 cooperate to define variable fluid volumes 96,98 sealed from eachother by dynamic seals 100 which, like seals 86, are preferably of theO-ring type but may be of the roll diaphragm type. In a manner analogousto pump 50, end caps 62,64 are secured and sealed to the housing memberin known manner.

Housing 88 includes axially aligned bores 88b,88c slidably supportingtie rod portion 38c for movement along an axis diametrical to cylinderbore 88a and piston 90. Piston 90 includes a flat slot 90c therethroughslidably receiving tie rod portion 38c and allowing to-and-fro movementof the piston normal to the tie rod portion. Piston 90 also includes ans-shaped cam slot 90d extending therethrough and receiving opposite endsof a pin cam follower 102 extending diametrically through tie rodportion 38c for effecting movement of the tie rod in response toto-and-fro or reciprocal movement of the piston and in accordance withthe shape of the cam slot.

Make-up valve and accumulator assemblies 66,68 include housings shownherein for illustrative purposes as one piece housings 108,110respectively. Accumulator housing 110 includes a cylindrical bore 110ahaving a free piston 112 slidably disposed therein and separating thebore into variable volume chambers 114,116 sealed from each other bydynamic seals 118. Chamber 114 contains a compressible fluid such as airand chamber 116 contains non-compressible oil of the type used in theclosed hydraulic system. A change valve 120 provides means for changingchamber 114 with air. A port 110b provides oil access to chamber 116.Accumulator 68 functions as a reservoir for the closed hydraulic systemand as such it need only provide pressure sufficient to ensure thesystem is fully charged. Several known devices are capable of thisreservoir function, e.g., a low pressure oil pump, a reservoir providinga head of pressure, a diaphragm type accumulator, etc.

As thus far described, conduit 54, pump volume 82 and actuator volume 98define a first closed fluid flow branch 104; and conduit 56, pump volume84 and actuator volume 96 define a second closed fluid flow branch 106.The branches are filed with a non-compressible hydraulic oil. Hence,sliding movement of pump piston 72 in response to steering movement ofthe front wheel steering tie rod 32 causes fluid flow in both branchesand a corresponding sliding movement of actuator piston 90 and rearwheel steering tie rod 38. The pistons move in proportion to theirsurface areas. Accordingly, rear wheel steering tie rod 38 moves inproportion to movement of piston 90 and in accordance with the profilesof the s-shaped cam 90d. In the disclosed embodiment of FIG. 2, thes-shaped cam profiles are selected to first move tie rod 38 indirections effecting same direction steering of the front and rearwheels and then opposite direction steering in response to increasedfront wheel steering angles. By way of example, maximum same andopposite direction steering angles of the rear wheels may be two degreesand six degrees, respectively. However, as may be seen, the cam profilesare readily changed to provide other same and opposite directionsteering angles. Further, the profiles may be selected to provide onlysame direction steering, opposite direction steering, and/or steeringangles that are not the same for left and right turns.

Make-up valve housing member 108 includes a cylindrical bore 108a havinga spool valving member 122 disposed therein, a port 108b in constantcommunication with the low pressure oil in accumulator chamber 116 via aconduit 124, and ports 108c,108d respectively in constant communicationwith closed hydraulic system branches 104,106 via make-up conduits54a,56a respectively connected to conduits 54,56. Valving member 122includes a stem portion 122a slidably and sealingly extending through anopening 108e in the housing end, an end pivotally connected to pumplever arm 74 by a pin 124, and axially spaced apart lands 122b,122c insliding, sealing contact with housing bore 108a.

System 22 is shown in the neutral position in FIGS. 1 and 2, i.e., thefront and rear wheels are being steered straight ahead, the pump andactuator pistons are axially centered in their respective bores, andvalving member lands 122b,122c allow free communication of oil betweenaccumulator chamber 116 and conduits 54,56, whereby branches 104,106 arefully changed with oil under relatively low pressure. When the frontwheels are steered leftward, front wheel tie rod end 32b movesrightward, thereby pivoting lever arm 74 clockwise about pin 76 toeffect rightward axial movement of make-up valve spool valving member122 and leftward axial movement of pump piston 72. Rightward movement ofspool valving member 122 causes land 122b to move to a position blockingport 108c. Leftward movement of pump piston 72 displaces oil from pumpchamber 82 to actuator chamber 98 to effect upward movement of actuatorpiston against the biasing force of spring 92 the the force needed tosteer the rear wheels, thereby causing the relatively low oil pressurein conduit 54 to quickly exceed the low oil pressure provided byaccumulator 68 and rise to an operational pressure suffice to steer therear wheels. Accumulator 68 and make-up valve 66 provide for replacementof any oil leakage in branches 104,106, and for oilexpansion/contraction due to temperature changes.

Looking now at a second closed hydraulic system embodiment 200, thereincomponents substantially identical to components in system 22 are giventhe same reference number with the addition of a prime suffix. System200 includes a pump assembly 50', an actuator assembly 202 having ports202a,202b analogous to ports 62a,64a of actuator 52, an accumulatorassembly 68', and a control assembly 205. The control assembly comprisesa steering direction valve 204, a make-up/shut-off valve 206, anelectronic control logic 208 which receives signals from a vehicle speedsensor 210 and a steering wheel position sensor 212. Actuator assembly202 differs from pump assembly 50' in that a piston 214 therein iscentered by springs 216,218, and a lever arm 220 thereof is mounted forpivotal movement about a pin 222 and is connected at its upper end 222ato rear wheel steering tie rod portion 38c' by a pin 224. Steeringdirection valve 204 and make-up/shut-off valve 206 respectively replacethe s-shaped cam and the make-up valve of FIG. 2.

Steering direction valve 204 includes ports 204a,204b respectivelyconnected to pump ports 60a',58a' via conduits 226,228, ports 204c,204drespectively connected to actuator ports 202a,202b via conduits 230,232,and a valving member 234 moved by a solenoid 236 between an oppositesteering direction position 234a and a same direction steering position234b. Make-up/shut-off valve 206 includes a port 206a connected toaccumulator port 110b' via a conduit 124' and ports 206b,206crespectively connected to conduits 230,232 via conduits 230a,232a, and avalving member 238 moved by a solenoid 240 between a position 238awherein system 200 is provided with make-up oil and is shut off orrendered inoperative and a position 238b wherein the system is madeoperative. Solenoids 236,240 are electrically connected to logic 208 viaa lines 242,244, respectively.

In a preferred mode of operation for system 200, logic 208 providessignals to solenoid 240 via line 244 for moving valving member 238 frommake-up/shut-off position 238a to position 238b when steering wheelposition sensor 212 indicates that the steering wheel has moved apredetermined distance from neutral or straight ahead steering of thefront wheels. A switch 246 may also be provided for selectivelymaintaining valve 238 in position 238a. Logic 208 also provides signalsto solenoid 236 for positioning valving member 234 in the oppositedirection steering position 234a when speed sensor 212 indicates avehicle speed less than a predetermined amount, e.g., twenty-two milesper hour (35 Km/hr). For vehicle speeds greater than the predeterminedamount, the logic moves valving member 234 to the same directionsteering position.

An alternative control assembly 300 comprises repositioningmake-up/shut-off valve assembly 206', optionally replacing two positionsteering direction valve assembly 204 with a three position steeringvalve assembly 302, and controlling valve assembly 302 as a function ofthrottle position rather than vehicle speed. More specifically,make-up/shut-off valve port 206a' is connected to accumulator 68 via aconduit 124', ports 206b',206c' are connected to conduits 226',228' viaconduits 226a,228a, respectively, and solenoid 240' is connected to andcontrolled by logic 304 via conductor line 244' in the same manner aswith control logic 208. Logic 304 receives steering wheel positionsignals from a position sensor 212' and switch 246'. Steering directionvalve assembly 302 includes ports 302a,302b respectively connected toconduits 226',228', ports 302c,302d respectively connected to conduits230',232', a valving member 306 moved by a solenoid assembly 308 betweenopposite and same direction steering positions 306a,306b analogous topositions 234a,234b, and a center position 306 for hydraulically lockingactuator piston 214 and inter-connecting conduits 226',228' for allowingfree movement of pump piston 72'. Control logic 304 provides signals viaconductor line 242' to solenoid assembly 308 to move or maintain valvingmember 306 in same direction steering position 306b for steering wheelpositions providing left and right front wheel steering angles up toabout one-quarter maximum steer angles, signals to the solenoid assemblyto move or maintain the valving member in opposite direction steeringposition 306a for steering wheel positions providing left and rightfront wheel steering angles greater than about three-eights maximumsteer angles, and signals to the solenoid assembly to move or maintainthe valving member in center position 306c for steering wheel positionsproviding left and right front wheel steering angles between aboutone-quarter and three-eights maximum.

Two embodiments of the invention have been disclosed therein forillustrative purposes. Many variations and modifications of thedisclosed embodiments are believed to be within the spirit of theinvention. For example, make-up valve 66 in system 22 may be employed inthe system 200 in lieu of valve 206. The following claims are intendedto cover the inventive features of the disclosed embodiments andvariations and modifications believed to be within the spirit of theinvention.

What is claimed is:
 1. A steering apparatus for a wheeled vehicle havingfront and rear dirigible wheel assemblies, the apparatuscomprising:first means for transmitting left and right steering movementto a front wheel steering assembly in response to a steering wheel beingturned left and right from a neutral position to effect left and rightsteering angles of the front wheels, second means for transmittingsteering movement to a rear wheel steering assembly in response tosteering movement of the front wheel steering assembly; the second meanscharacterized by: a closed hydraulic system including first and secondclosed fluid flow branches comprising a fluid displacement assemblyoperative to effect fluid flow in both branches in response to steeringmovement of the front wheel steering assembly; a hydraulic actuatorassembly operative to effect steering movement of the rear wheelsteering assembly in response to said fluid flow, the fluid in thebranches alternately increasing from a low pressure to an operationalpressure in response to alternate steering movement of the front wheelsteering assembly; a source of fluid under pressure less than theoperational pressure in the closed branches; and a control assemblyincluding a valve assembly having a first port connected to the fluidsource, second and third ports respectively connected to the first andsecond branches, a valving member movable to a first position inresponse to the neutral position of the steering wheel and movable to atleast a second position in response to a non-neutral position of thesteering wheel, the valving member first position communicating thefirst port with the second and third ports and the second positionblocking at least the communication to the port connected to the branchincreasing to operational pressure.
 2. The steering apparatus of claim1, wherein the valving member is movable to the second position inresponse to turning the steering wheel in one direction from the neutralposition and to a third position in response to turning the steeringwheel in the other direction from the neutral position, the valvingmember second and third positions respectively operative to block theone of the second and third ports connected to the branch increasing tooperational pressure.
 3. The steering apparatus of claim 2, wherein thevalve assembly includes means mechanically connecting the valving memberto a portion of the front wheel steering apparatus which moves inopposite directions in response to opposite steering directions.
 4. Thesteering apparatus of claim 1, whereinthe fluid displacement assemblyand the hydraulic actuator assembly each including a reciprocal pistondisposed in a bore of a housing with one end of each bore connectedtogether by a first conduit means and the other end of each boreconnected together by a second conduit means, whereby reciprocalmovement of the displacement assembly piston effects a proportionalreciprocal movement of the actuator assembly piston; and cam meansdisposed between the actuator piston and the rear wheel steeringassembly for controlling the rear wheel steering angle according to theprofile of the cam means and according to the distance the actuatorpiston moves.
 5. The steering assembly of claim 4, wherein the profileof said cam assembly moves the valving member in a direction effectingsteering movement of the rear wheel steering assembly in the samedirection of the steering direction of the front wheel steering assemblyfor front wheel steering angles below a predetermined amount and inopposite directions as the steering direction of the front wheelassembly for front wheel steering angles greater than the predeterminedamount.
 6. The steering apparatus of claim 1, wherein the fluiddisplacement assembly and the hydraulic actuator assembly each includinga reciprocal piston disposed in a bore of a housing defining a first andsecond variable fluid volume at each end of the piston, the assembliesincluding means for mechanically connecting the displacement andactuator pistons respectively to the front and rear wheel steeringassemblies; the first and second fluid flow branches respectivelyincluding one volume of each assembly; and the control assemblyincluding a rear wheel steering direction valve interposed in thebranches between the assemblies, and having a valving member movable toa same direction position interconnecting the first and seconddisplacement assembly volumes respectively with the first and secondactuator assembly volumes for effecting same direction steering of thefront and rear wheels and movable to an opposite direction positioninterconnecting the first and second displacement assembly volumesrespectively with the second and first actuator assembly volumes foreffecting opposite direction steering of the front and rear wheels. 7.The steering apparatus of claim 6, wherein the control assemblyincludes:actuation means operative to move the valving member of thedirection valve to the same direction position for front wheel steeringangles less than a first predetermined amount and then to the oppositedirection position for front wheel steering angles greater than a secondpredetermined amount.
 8. The steering assembly of claim 7, wherein thefirst and second predetermined amounts are the same.
 9. The steeringapparatus of claim 7, wherein the actuation means is operative to movethe valving member of the direction valve to a third positioninterconnecting the displacement assembly volumes and hydraulicallylocking the actuator assembly volumes in response to front wheelsteering angles between the first and second positions.
 10. The steeringapparatus of claim 6, wherein the control assembly includes:actuationmeans operative to move the valving member of the direction valve to thesame direction position in response to vehicle speed being less than afirst predetermined amount and to the opposite direction position inresponse to vehicle speed being greater than a second predeterminedamount.
 11. The steering assembly of claim 10, wherein the first andsecond predetermined amounts are the same.
 12. The steering apparatus ofclaim 10, wherein the actuation means is operative to move the valvingmember of the direction valve to a third position interconnecting thedisplacement assembly volumes and hydraulically locking the actuatorassembly volumes in response to vehicle speed being between the firstand second amounts.